Focused ion beam (FIB) systems are used in a variety of applications in integrated circuit manufacturing and nanotechnology to create and alter microscopic and nanoscopic structures. FIB systems can use a variety of sources to produce ion, such as a plasma source or a liquid metal ion source (LMIS). A LMIS can provide high resolution processing, that is, a small spot size, but typically produces a low beam current. Until recently, plasma systems were known to use higher beam currents, and thus allow faster material processing, but were incapable of being focused to a spot size small enough for working with nanoscale structures. More recently, however, inductively coupled plasma (ICP) sources have been developed which are capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a small spot. Such an ICP source is described by Keller et al., in “Magnetically Enhanced, Inductively Coupled Plasma Source for a Focused Ion Beam System,” U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention and incorporated herein by reference.
When using conventional focused ion beam (FIB) based systems, it is often the case that the system will be used, but then will need to remain idle for some period of time before the ion beam is needed again. In this situation, it is customary to “blank” the beam, rather than shut down the beam altogether. This is largely because of the settling time required when the beam is turned back on, which in some cases may be as long as an hour. Beam blanking is typically accomplished by way of an electrostatic “beam blanker,” which can consist of metal plates (from ˜5 mm to 10 cm in length) in the microscope column with a small gap (˜0.25 to ˜2 mm) where the beam passes, which will deflect the beam when a voltage (from ˜5 v to ˜400 v) is applied to one plate, while the other is kept at ground potential. The beam blanker is located immediately upstream from an aperture plate having a small opening for the beam. The beam is blanked (prevented from reaching the sample) when the voltage is applied to the beam blanker because the deflection of the beam makes it miss the opening in the subsequent aperture plate. This results in the beam striking the aperture plate or even the beam blanking plates.
When the ion beam is blanked, it is thus prevented from striking the sample (and causing unintended damage to the sample) but the beam is still being generated and causing wear in the column. The beam blanking plates and the aperture plate, which the beam is striking while blanked, are themselves slowly destroyed by the ion beam. Many FIB systems also include other apertures in the column above the blanking plane (such as a beam acceptance aperture) which are also being struck and slowly destroyed while the beam is in operation.
Even though beam blanking causes some degree of destruction of the apertures, as well as other wear on the column itself, it has typically been seen as more desirable that turning off the ion source completely. In a LMIS, when the ion source is completely turned off, the ions tend to go completely cold thermally. For a plasma source, the source is usually turned off by extinguishing the plasma. In either case, turning the beam back on involves some period of settling time before the tool will operate reliably. In some cases, this settling time can be as much as an hour. Thus, there is a predisposition among operators of prior art FIB systems to leave the FIB systems on when not used because of the lengthy amount of time required to turn the systems off, back on, and then to return to an operational steady-state.
What is needed therefore is an improved method and apparatus for shutting down an ion beam, whether in a LMIS or ICP system, that does not cause damage to the apertures above the blanking plane and that does not require an unacceptably long time for the system to settle once the ion beam is restored.